1. Field of the Invention
The present invention relates to a braking force distribution control device, and in particular, to a braking force distribution control device which is suitably used in braking control of a vehicle. The braking force distribution control device of the present invention can be used as a control torque control device for controlling braking torque, or as a braking force distribution control device for controlling the braking forces distributed to the respective wheels of a vehicle.
2. Description of the Related Art
In order to control the braking forces of the respective wheels of a vehicle, it is necessary to exhibit, to the maximum extent possible, the braking abilities of the respective wheels, while appropriately maintaining the posture (yaw motion) of the vehicle. However, considering braking during straight line traveling and braking during turning, the deceleration ability and the vehicle stability have an inverse relationship. Namely, although the deceleration ability is improved if the braking forces of the rear wheels are increased, conversely, the vehicle stability is sacrificed because the cornering forces of the rear wheels decrease.
In order to have a vehicle exhibit both of these contradictory properties, Japanese Patent Application Laid-Open (JP-A) No. 5-278585 discloses a device which controls the brake fluid pressures of the rearwheels on the basis of the wheel slip. Further, JP-A No. 63-13851 discloses a device for adjusting the left and right braking forces on the basis of the state of turning.
However, in these techniques, because the braking forces of the wheels are controlled without judging the grip forces of the tires at the time of braking, a problem arises in that, as the braking ability improves, the vehicle stability deteriorates. Further, although the braking force of each wheel differs in accordance with the traveling state of the vehicle, an equivalent braking torque is applied to each wheel in the aforementioned techniques. Thus, depending on the traveling state of the vehicle, the vehicle stability might deteriorate extremely.
Further, in order to exhibit, to the maximum extent possible, braking ability at each wheel while maintaining an appropriate posture of the vehicle, JP-A No. 6-16117 discloses a technique as follows. The sum total of the longitudinal force applied to the vehicle (the total longitudinal force) is detected or set, and an allotted load ratio of each wheel is determined. By distributing the total longitudinal force to the respective wheels in accordance with the allotted load ratios, target wheel longitudinal forces which are to be applied to the respective wheels are set, and the longitudinal forces of the respective wheels are controlled on the basis of these target wheel longitudinal forces.
Further, JP-A No. 4-224449 discloses a technique in which the brake fluid pressure is controlled such that values, which are obtained by dividing the braking forces of the front and rear wheels by the loads of the front and rear wheels respectively, are equal.
However, in this conventional technique, control of the braking force is not carried out in consideration of the frictional state between the road surface and the tire. For example, even if a wheel having a large load is on a low μ (friction) road, a problem arises in that an excessive braking force is distributed to that tire, such that there is the possibility that the tire may slip.